An internal combustion engine like an automobile is provided with a camshaft driven by the engine in synchronism therewith, a cam fixedly secured to the camshaft, and a rocker arm disposed between the cam and a valve stein of an intake (or exhaust) valve rockably about a fulcrum provided by a hydraulic lash adjuster. In this case, a driving force of the camshaft is transmitted to the valve stem via the cam and the rocker arm to displace the valve stein to thereby close/open the intake/exhaust valve. The hydraulic lash adjuster is adapted to automatically regulate a valve clearance between the cam and the rocker arm in response to an (extension/retraction) operation of the lash adjuster.
Provided in such a hydraulic lash adjuster as described above is a hollow plunger slidably fitted in a bottomed cylindrical body with a plunger's head portion extending from an open end of the body, as disclosed in Patent Document 1. The plunger is provided in its hollow section with a low-pressure chamber for storing oil, a communication hole formed in the plunger's head portion for communication of the low-pressure chamber with its exterior, and a high-pressure chamber to be charged with oil and located between the bottom of the body and the bottom of the plunger in the body. Each of the peripheral walls of the plunger and the body has an oil supply hole constituting an oil supply passage to supply oil to the low-pressure chamber. A valve mechanism is provided at the bottom of the plunger adapted to open a valve by a restoring force of a biasing means disposed between the bottom of the body and the bottom of the plunger to thereby allow for a flow of oil from the low-pressure chamber to the high-pressure chamber when the plunger is push-moved by the biasing means.
Use of this hydraulic lash adjuster in a valve operating mechanism of an internal combustion engine makes it possible to rockably support the rocker arm on the plunger's head portion and generate a damping force when an input load from the rocker arm is received by a hydraulic pressure of the high-pressure chamber and the plunger is retracted. On the other hand, in a case where a valve clearance takes place, the plunger is extended (in the direction of extension) to annihilate the valve clearance.
It is noted that in such a hydraulic lash adjuster as stated above, oil pumped by an oil pump from an oil pan is supplied to the high-pressure chamber via an oil passage and the low-pressure chamber during operation of the engine. Meanwhile, air bubbles are sometimes taken into the oil on the way to the high-pressure chamber. If the air bubbles remain in the high-pressure chamber, they can be easily compressed by the retracting plunger, resulting in easy contraction of the plunger which impedes a basic function of the plunger to generate a damping force.
For this reason, in recent years in order to prevent air bubble from infusing into the high-pressure chamber, a proposition has been made to provide in the plunger a guide cylinder for guiding the oil that flows from the oil supply passage into the plunger to the plunger's head portion via a space between the outer periphery of the guide cylinder and the inner periphery of the plunger, as disclosed in Patent Document 2. Specifically the guide cylinder is formed with a portion having at its leading end a smaller outer diameter than that of a base section of the guide cylinder. The leading end of the guide cylinder is oriented towards the plunger's head portion and the outer periphery of the base section of the guide cylinder is engaged with an inner periphery portion of the plunger nearer to the plunger bottom than to the oil supply passage, thereby forming an annular space between the inner periphery of the plunger and the outer periphery of the guide cylinder at a position nearer to the plunger's head portion than to the oil supply passage. Thus, during operation of the engine, the oil supplied from the oil supply passage is first guided towards the plunger's head portion through the annular space, where air bubbles are gradually separated from the oil (growth of air bubbles). While the oil itself flows back to the high-pressure chamber via the guide cylinder when the valve is opened by the valve mechanism, the separated air is discharged out of the plunger from the communication hole of the plunger's head portion together with surplus oil.